ASK AMP MAN
Impedance Mismatch and Vox Mods
Hi Jeff, amp into the 16-ohm stereo input will more
I have my amp speaker output set to 16 than likely only engage one speaker in the
ohms and I’m running into the 8-ohm mono cabinet, as the cabinet would then be config-
input of a 2x12 cabinet, which also has a ured for left/right stereo operation. I wouldn’t
stereo input of 16 ohms. The cabinet has recommend placing the full output of a 100-
two 16-ohm speakers wired in parallel for watt amp on only one speaker.
a total of 8 ohms. Will I damage my amp
running the 16-ohm from the amp into the
8-ohm cabinet? Should I run it into the 16-
ohm stereo input instead? My current configuration sounds a lot better than matching
the impedance – I’m assuming this is caus- Clay
ing the amp to run hotter.
The amp is 100 watts and I
rarely take the master volume
above 3. I like the way this
sounds, but I’m afraid I might
be causing damage to the
amp.
Jeff,
In a Vox AC30 can the Cut control be mod-
ded to be a post-PI master volume?
two legs of the phase inverter are electrically
brought closer together through the capacitor, causing just the high frequencies to be
canceled. The larger the capacitor value, the
wider the frequency range that is canceled
becomes. If the capacitor was replaced with a
wire, this would cause the entire range of frequencies to be cancelled as the pot is rotated
and the resistance between the two legs is
decreased. Once the resistance reaches zero,
all frequencies are canceled and there would
be no volume, hence a master volume control.
Paul
Hi Paul,
The short answer to your ques-
tion is yes, you may damage
your amp if you continue to
run it this way. You don’t men-
tion the particular make of the
amplifier, but some amps seem
to be more susceptible to dam-
age from this type of mismatch
than others. Regardless, this
particular type of impedance
mismatch is not recommended.
When the speaker load is
greater (has a lower number) than the output
impedance of the amp, the load placed on
the output tubes increases and can cause
premature failure of the tubes. Tube failure
can sometimes cause other component failure
in the amp as well – screen grid resistors and
hum balance resistors are typical casualties of
shorted output tubes.
There are several ways to
turn your AC30’s cut control
into a master volume
Hi Clay,
There are a few different styles of post-phase
inverter (PI) master volume controls that can
be installed in most amplifiers, and while most
require dual pots and additional resistors and
capacitors, there is also a very simple style
that can be employed using the single pot in
the Vox “cut” circuit.
You could make this a permanent
master volume control by simply
replacing or shorting across the
“cut” capacitor with a wire, but
you could also install a switch to
engage or disengage the short,
enabling the control to be used
as either a Cut control or master
volume. I will tell you, however,
that once you engage the short
across the capacitor, the output
level of the amp will more than
likely change. Even with the
Cut control set to maximum
resistance there will still be 250K
ohms of resistance across the
phase inverter outputs, reducing
the signal level to some degree.
This may not pose a problem for
you though because, hey, isn’t
the point of a master volume
control to reduce the output
level anyway? If it does indeed
pose an issue, you might want to consider
increasing the value of the pot in order to
minimize the loading effect on the circuit.
You could even utilize a pot with a push-pull
switch so that you could switch between
a master volume and cut function. Just be
aware that if you change the value of the pot,
the cut circuit will then perform differently.
Generally, an impedance mismatch is chosen
so that the output power, due to the mis-
match, is reduced, causing the output stage
to break up or distort earlier, providing the
warm, overdriven, tube-like tone that we
all know and love at a lower volume level.
If you’d like to employ a mismatch for this
reason, the safer method would be to set the
output impedance to 4 ohms instead of 16.
Regarding the speaker cabinet, running the
The Cut control functions by electrically connecting part of the output signal of each half
of the phase inverter together. Because the
signal of each half of the phase inverter is
out of phase with its counterpart, connecting
them together causes them to cancel each
other out. The relatively small capacitor value
(.0047µf) in series with a 250K ohm pot used
in the cut circuit ensures that as the control
is rotated and its resistance decreased, the
Enjoy and remember, just like life, it’s all
about compromise.
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2008 Jeff Bober
